Stainless steel typically requires a stabilization treatment where such material is used at operating temperatures above 900° F. (482° C.). In many cases, the stabilization treatment includes a 1650° F. (899° C.) heating step after fabrication. However, at operating temperatures above 900° F. (482° C.), stabilization treatment tends to compromise the high temperature service weld and heat affected zone (HAZ) integrity through sigma phase embrittlement. Moreover, and especially at relatively high temperatures, stabilization treatment also reduces impact properties, elevated temperature creep properties, and/or increases susceptibility to reheat cracking.
There are various mitigation techniques known in the art to overcome at least some of the problems associated with stabilization treatment. However, current experience seems to indicate that susceptibility to cracking cannot be entirely eliminated. For example, use of 347 type stainless steels in high temperature operating environments is generally limited by reheat cracking during post weld heat treatment (PWHT) and/or stress relaxation cracking after long-term elevated temperature service.
Commonly, known heat treatments include thermal stress-relief to reduce residual stresses, solution-annealing to dissolve carbides, ferrite and sigma, and heat stabilization to form carbon adducts (e.g., chromium carbide precipitates) with alloy components.
Stress Relief: Optimal time and temperature for stress relief are reported between 1550° F. and 1650° F. (843° C. and 899° C.) for about 2 hours. Commonly, stress relief PWHT is performed on TP 347 stainless steel piping between 1550° F. and 1650° F. (843° C. and 899° C.) to reduce residual stresses from cold working and/or joint restraints, and to further reduce the susceptibility to chloride stress corrosion cracking.
Solution Annealing: In most cases, solution annealing relieves all or almost all of the welding related residual stresses, dissolves chromium carbides, converts delta ferrite to austenite in equilibrium phase-fractions, and/or spheroidizes the remaining ferrite, thus imparting corrosion resistance comparable to the base metal. It is generally recommended to perform solution annealing relatively quickly (e.g., less than 60 minutes) to minimize oxidation and surface chromium depletion. Depending on the alloy, solution annealing is generally performed at 1900° F. to 2000° F. (1038° C. to 1093° C.) in most cases.
Stabilization Heat Treatment: Stabilization heat treatment is thought to dissolve nearly all remaining chromium carbides (Cr23C6) that segregated at the grain boundaries from previous heat treatments or thermal operations (e.g., welding). Stabilization heat treatment is also thought to provide stress relief and is sometimes referred to as stabilization anneal. In most known applications, stabilization is performed by heating at 1650° F. (899° C.) for up to 4 hours followed by air cooling to ambient temperature to minimize sensitization.
Unfortunately, the stabilization heat treatment can also lead to substantial degradation of mechanical and corrosion properties because of complex physical-chemical interactions. For example, currently practiced stabilization heat treatment at 1650° F. (899° C.) frequently maximizes the rate of fine niobium carbide formation and allows for sigmatization of most remaining ferrite, often leading to substantial loss of ductility and elevated-temperature creep strength. Therefore, to prevent failure during high temperature service, heat treated stainless steel use is generally limited to uses with operating temperatures below 950° F. (510° C.) to ensure immunity to sensitization.
In further known processes, additional heat treatments may be included as described in U.S. Pat. No. 4,418,258 to McNealy et al. to improve structural integrity. McNealy's heat treatments significantly improve resistance to cracking and corrosion, however, are generally limited to low-alloy materials (i.e., materials with less than 5% alloying metals). In other known methods, as described in U.S. Pat. No. 6,127,643 to Unde, certain welding processes are employed to control the cooling process of a weld. While Unde's welding process tends to reduce at least some of the problems associated with numerous cooling gradients in a weld (e.g., crystalline inhomogeneity, etc.), various problems nevertheless remain. Among other things, Unde's process will in many cases provide only limited use for stainless steel.
Therefore, while rapid progress in elevated temperature petrochemical technology has created a demand for use of stainless steels beyond the traditional operating limits of 950° F. (510° C.), existing heat treatments typically fails to eliminate problems associated with loss of ductility, creep strength, and/or cracking. Thus, there is still a need to provide improved methods and compositions for stainless steel.